I have a menu in the original language English. Spanish is active in admin/config/regional/language . Custom menu link is active in admin/config/regional/content-language . I now have a Translation tab in admin/structure/menu/manage/mymenu/translate. I click Add in the Spanish row, change the label and Description (only Description is sufficient). Then what? How do I add menu items to the now supposedly created Spanish version? I tried several things but could not figure out where this new (empty?) Spanish menu is? It would be logical and consistent when now an "Add link" button would also appear in admin/structure/menu/manage/mymenu/translate/es/edit like in admin/structure/menu/manage/mymenu.
What is missing is some simple help text right in the forms that guide our thoughts in the right direction. For instance what is the purpose of the "Menu language" select box? Is that to change the language of the whole menu? But wasn't the language fixed already at creation (the default language)? I could not find a decent explanation so I resort to the help of somebody who managed to got it working.